1. Field of the Invention
This invention relates to using a multi-phase pulse modulation technique to implement a wideband polar transmitter, and more particularly, to a multi-phase pulse-modulated polar transmitter and a method of generating pulse-modulated envelop signals carrying modulated RF signal that may enhance the efficiency and bandwidth of a power amplifier and reduce out-of-band emissions.
2. Description of Related Art
In general, a power amplifier has long been known as the most power-consuming component in a radio-frequency (RF) transmitter. By improving the power amplifier efficiency, the overall battery life can be greatly increased. The envelope elimination and restoration (EER) method, which was first proposed in “Single-sideband transmission by envelope elimination and restoration” (Proc. of the IRE, pp. 803-806, July 1952, L. R. Kahn), may efficiently amplify RF signals. The EER method splits the RF signal into a constant-envelope modulated signal and an envelope signal, amplifies the constant-envelope signal, and restores the signal based on the envelope signal, respectively, such that high overall power efficiency may be attained.
The EER method is implemented in “An envelope elimination and restoration power amplifier using a CMOS dynamic power supply circuit” (IEEE MTT-S Int. Microwave Symp. Dig., vol. 3, pp. 1591-1522, June 2004, J.-H. Chen, K. U-Yen, and J. S. Kenney) and “L-band transmitter using Kahn EER technique” (IEEE Trans. Microwave Theory Tech., vol. 46, no. 12, pp. 2220-2225, December 1998, F. H. Raab, B. E. Sigmon, R. G. Myers, and R. M. Jackson), in which a highly efficient power supply circuit is used to dynamically modulate the drain or collector of a non-linear, but highly-efficient RF power amplifier. One of the major problems in implementing a Kahn EER transmitter is to synchronize the phase and envelope paths. The non-synchronization of the phase and envelope paths may create unwanted the out-of-band emissions. Conventional EER transmitters use an L-C low-pass filter to filter the output of the switching envelope amplifier or dynamic bias circuit. The low-pass filter, however, creates significant delay that requires the phase data to be delayed accordingly using shift registers or delay circuits. For these transmitters, the phase information may require as many as 16 bits per clock for storage. Using the shift registers to store such data may adversely increase the area and cost of the transmitter.
Pulse modulation of the input signals using sigma-delta modulation (Σ-Δ Modulation) for implementing a Kahn EER transmitter has been proposed in “An improved Kahn transmitter architecture based on Delta-Sigma Modulation” (IEEE MTT-S Int. Microwave Symp. Dig., vol. 3, pp. 1327-1330, June 2004, Y. Wang), “An EER transmitter architecture with burst-width envelope modulation based on triangle-wave comparison PWM” (Proc. IEEE Int. Symp. PIMRC, pp. 1-5, September 2007, M. Taromaru, N. Ando, T. Kodera, and K. Yano) and “A transmitter architecture for nonconstant envelope modulation” (IEEE Trans. Circuit and Syst. II, vol. 53, no. 1, pp. 13-17, January 2006, C. Berland, I. Hibon, J. F. Bercher, M. Villegas, D. Belot, D. Pache, and V. Le Goasccoz). The main benefit of using this technique is the omission of the low-pass filter in the envelope path. By omitting the low-pass filter, the delay in the envelope path is reduced significantly, and hence synchronizing the phase and envelope paths using shift registers may be simplified. However, the use of the EER method requires high quality band-pass filters to filter the unwanted out-of-band emissions created by pulse modulation or SDM noises, which hampers the wide use of the EER method. Pulse modulation of the input signals using pulse-width modulation (PWM) was shown in “A transmitter architecture for nonconstant envelope modulation” (IEEE Trans. Circuit and Syst. II, vol. 53, no. 1, pp. 13-17, January 2006, C. Berland, I. Hibon, J. F. Bercher, M. Villegas, D. Belot, D. Pache, and V. Le Goasccoz), but was concluded as unattractive since spurious emissions are too high.
The aforesaid conventional EER transmitters require the use of envelope detectors and RF limiters to obtain the constant-envelope modulated RF signal. Thanks to the advancement of modern digital signal processing technology, it is now possible to directly generate the phase and envelope signals digitally to implement a polar transmitter. The polar transmitter may be applied in cellular phones for CDMA2000 and W-CDMA standards. In the related applications, a high quality bass-pass filter is required at the output end of the transmitter to suppress the unwanted out-of-band emissions generated from the PWM. The modern cellular phones supporting CDMA2000 and W-CDMA standards have high quality surface-acoustic-wave (SAW) filters between the antennas and the power amplifiers. However, unless the PWM sampling frequency is sufficiently high, the SAW filter alone is not adequate to suppress the unwanted out-of-band emissions.
Therefore, how to provide a polar transmitter that may resolve the drawbacks of the prior art, to suppress the out-of-band emissions generated from the PWM and enhance the signal bandwidth and efficiency of the power amplifier, is crucial for handset power amplifier development.